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North Carolina CDL: Air Brakes

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North Carolina CDL Air Brakes

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Question 1 of 30
  1. 1. The assigned truck has an air compressor with a separate oil supply. Its engine-oil check is satisfactory. Which item still belongs in the pre-trip check?
  2. 2. An equipment checklist identifies this truck's air compressor as belt-driven. Which observation is specifically relevant to that drive arrangement?
  3. 3. A stationary truck's reservoir pressure reaches its specified upper setting and stops rising while the engine continues to run. Later, the compressor resumes adding air at the specified lower setting. What does this sequence demonstrate?
  4. 4. In a safely secured training vehicle, the compressor has stopped, but the charged air system can still make brake applications. What accounts for this?
  5. 5. At the end of a driving day, a truck with manual-only reservoir drains shows normal air pressure. Why does that reading not justify skipping the specified draining?
  6. 6. A truck's automatic reservoir drain includes an electric heater. What is that heater intended to protect against in cold weather?
  7. 7. This vehicle is equipped with an approved alcohol evaporator and manual-only reservoir drains. Its evaporator has been serviced according to the vehicle instructions. What follows?
  8. 8. During an air-system check, the reservoir safety relief valve releases air because pressure has become excessive. What is the appropriate conclusion?
  9. 9. While waiting in traffic, a driver needlessly presses and releases an air-brake pedal over and over. Why can this reduce the available braking reserve?
  10. 10. In a cutaway of an S-cam drum brake, the chamber pushrod moves the slack adjuster and turns the camshaft. What should the turning S-cam do next?
  11. 11. A technician identifies the wheel brake being inspected as an air-operated disc brake, not a drum brake. Which friction arrangement should the trainee expect?
  12. 12. A truck has reservoir supply gauges and a separate application-pressure gauge. With the service pedal released, the supply gauges show charged tanks but the application gauge is near zero. How should these readings be understood?
  13. 13. Under comparable driving conditions, increasingly high application pressure is needed to hold the same speed. Which assessment is justified?
  14. 14. A training vehicle uses the air-pressure-operated electrical switch described in the manual for its stop lamps. What event should operate that switch?
  15. 15. A cutaway shows a conventional spring-brake chamber in its released driving condition. What is holding the powerful parking spring back?
  16. 16. A truck's foundation brakes are known to be out of adjustment. Someone argues that its strong parking springs make that defect irrelevant to parking and emergency braking. What is the flaw?
  17. 17. A driver familiar with a newer yellow push-pull parking knob is assigned an older air-braked truck with a labeled parking lever. What should guide the driver's use of it?
  18. 18. A straight truck has a dual air-brake system but only one service-brake pedal. A trainee thinks a second pedal must be missing. Which explanation resolves the concern?
  19. 19. Before departure in a dual-air straight truck, one reservoir reading is normal but the other remains below the vehicle's required operating pressure. What should the driver do?
  20. 20. A pre-trip check identifies excessive brake pushrod travel on a truck equipped with automatic slack adjusters. An unqualified driver proposes turning the adjuster until the reading looks acceptable. What is the better response?
  21. 21. During the walk-around, a hose connected to a brake chamber has been rubbing against a bracket and is visibly worn. No hiss is heard at that moment. How should it be treated?
  22. 22. A correctly conducted, safely secured pre-trip check confirms that a truck's low-air warning does not operate as specified. The supply gauge itself works. What conclusion follows?
  23. 23. A dual-air truck has larger-than-minimum reservoirs. Its manufacturer specifies a buildup time appropriate to that configuration. How should a trainee assess its pressure buildup?
  24. 24. In the manual's safely secured reservoir-leakage check, the engine is shut off before the pressure loss is measured. What is the reason for measuring without the compressor adding air?
  25. 25. A completed check shows that a truck cannot build air pressure fast enough to meet the specification for its actual system. The route is short. What is the appropriate decision?
  26. 26. Under the prescribed safe test conditions, a truck remains stationary during a gentle low-gear pull against its applied parking brake. What has that check established?
  27. 27. A driver has already recognized a hazard and pressed the pedal, but time is still needed for air to travel through the system and apply the wheel brakes. Which part of stopping is being described?
  28. 28. A truck's drum brakes have become excessively hot from overuse, although reservoir pressure remains normal. Why can stopping effectiveness still decline?
  29. 29. Some brakes on a truck are out of adjustment and contribute less than their share during repeated braking. What additional problem can develop at the brakes still doing most of the work?
  30. 30. While an air-braked straight truck is moving, its low-air warning comes on and pressure is falling. Service braking is still available. What is the safest objective?
Review all 30 questions with answers

  1. The assigned truck has an air compressor with a separate oil supply. Its engine-oil check is satisfactory. Which item still belongs in the pre-trip check?

    Correct answer: The oil level in the compressor's separate supply

    The manual distinguishes a compressor's own oil supply from engine-supplied lubrication. A satisfactory engine-oil reading does not establish the level in the separate compressor supply.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.1 - Air Compressor; printed5-1, physical77. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  2. An equipment checklist identifies this truck's air compressor as belt-driven. Which observation is specifically relevant to that drive arrangement?

    Correct answer: Whether the compressor belt is sound and properly tensioned

    A belt-driven compressor needs a belt-condition and tightness check. The qualification matters: some compressors are gear-driven, so this is not a universal compressor-belt requirement.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.1 - During Step 2 Engine Compartment Checks; printed5-6, physical82. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  3. A stationary truck's reservoir pressure reaches its specified upper setting and stops rising while the engine continues to run. Later, the compressor resumes adding air at the specified lower setting. What does this sequence demonstrate?

    Correct answer: The governor is controlling compressor pumping

    The governor controls when the compressor pumps into the reservoirs. Cut-out and cut-in are pressure-based operating points; their actual values must be checked against the manufacturer's specifications.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.2 - Air Compressor Governor; corroborating 5.3.3 on physical83/printed5-7; printed5-1, physical77. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  4. In a safely secured training vehicle, the compressor has stopped, but the charged air system can still make brake applications. What accounts for this?

    Correct answer: The reservoirs retain a limited supply of compressed air

    Reservoirs store compressed air, so applications can remain possible after the compressor stops. The stored supply is finite; this is not permission to drive with a failed compressor.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.3 - Air Storage Tanks; printed5-1, physical77. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  5. At the end of a driving day, a truck with manual-only reservoir drains shows normal air pressure. Why does that reading not justify skipping the specified draining?

    Correct answer: Water and oil can remain in tanks that show normal pressure

    A supply gauge shows pressure, not whether water and compressor oil have collected at the bottom of a reservoir. The manual calls for end-of-day drainage on systems without automatic drains.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.4 - Air Tank Drains; corroborating 5.4.8 on physical85/printed5-9; printed5-1, physical77. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  6. A truck's automatic reservoir drain includes an electric heater. What is that heater intended to protect against in cold weather?

    Correct answer: The automatic drain freezing

    Some automatic drains have electric heaters to help prevent the drain from freezing. That is a drain-system feature, not a means of warming friction linings or controlling governor settings.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.4 - Air Tank Drains; printed5-1, physical77. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  7. This vehicle is equipped with an approved alcohol evaporator and manual-only reservoir drains. Its evaporator has been serviced according to the vehicle instructions. What follows?

    Correct answer: The specified reservoir-draining routine still applies

    On a system fitted with an alcohol evaporator, its ice-prevention role does not remove collected water and oil from manually drained reservoirs. This does not advise adding alcohol to a system that is not designed for it.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.5 - Alcohol Evaporator; printed5-2, physical78. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  8. During an air-system check, the reservoir safety relief valve releases air because pressure has become excessive. What is the appropriate conclusion?

    Correct answer: The pressure-control fault needs qualified attention

    The safety valve protects the system from excessive pressure. Its release is a warning of a fault to be corrected, not evidence of a completed brake test or a reason to alter its setting.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.6 - Safety Valve; printed5-2, physical78. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  9. While waiting in traffic, a driver needlessly presses and releases an air-brake pedal over and over. Why can this reduce the available braking reserve?

    Correct answer: Each release exhausts air that the compressor must replace

    Air is exhausted as service brakes release. Unnecessary repeated applications can use air faster than the compressor replaces it. This is a warning against needless cycling, not an instruction for emergency braking.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.7 - The Brake Pedal; printed5-2, physical78. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  10. In a cutaway of an S-cam drum brake, the chamber pushrod moves the slack adjuster and turns the camshaft. What should the turning S-cam do next?

    Correct answer: Spread the shoes against the inside of the drum

    In the S-cam design, cam rotation moves the brake shoes outward against the drum. The question concerns the illustrated mechanism, not an adjustment procedure or every air-brake design.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.8 - Foundation Brakes > S-cam Brakes; Figure5.2; printed5-2, physical78. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  11. A technician identifies the wheel brake being inspected as an air-operated disc brake, not a drum brake. Which friction arrangement should the trainee expect?

    Correct answer: A rotor gripped between pads in a caliper

    Disc brakes slow the wheel by clamping a rotor between friction pads. This item does not generalize the manual's older actuator description to every current air-disc brake model.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.8 - Foundation Brakes > Disc Brakes; printed5-2, physical78. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  12. A truck has reservoir supply gauges and a separate application-pressure gauge. With the service pedal released, the supply gauges show charged tanks but the application gauge is near zero. How should these readings be understood?

    Correct answer: Stored air pressure can be present without a service-brake application

    Supply gauges show pressure available in the reservoirs; an application gauge shows pressure being applied to the brakes. They measure different things, so their readings need not match.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.9 - Supply Pressure Gauges; 5.1.10 - Application Pressure Gauge; printed5-2, physical78. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  13. Under comparable driving conditions, increasingly high application pressure is needed to hold the same speed. Which assessment is justified?

    Correct answer: Fade, maladjustment, leaks or other faults are possible

    The manual associates this change with fade and also lists maladjustment, air leaks and other mechanical problems. A rising application reading must not be treated as proof of better brakes or as a unique diagnosis.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.10 - Application Pressure Gauge (continued from physical78/printed5-2); printed5-3, physical79. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  14. A training vehicle uses the air-pressure-operated electrical switch described in the manual for its stop lamps. What event should operate that switch?

    Correct answer: Air pressure applying the service brakes

    In this described arrangement, brake-application air pressure operates an electrical switch that turns on the stop lamps. This is not a claim that every vehicle uses the same wiring or switch design.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.12 - Stop Light Switch; printed5-3, physical79. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  15. A cutaway shows a conventional spring-brake chamber in its released driving condition. What is holding the powerful parking spring back?

    Correct answer: Air pressure acting against the parking spring

    Conventional spring brakes are held released by air pressure. Removing that pressure lets the powerful spring apply the brake; service-brake air application and spring-brake release must not be confused.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.14 - Spring Brakes; printed5-3, physical79. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  16. A truck's foundation brakes are known to be out of adjustment. Someone argues that its strong parking springs make that defect irrelevant to parking and emergency braking. What is the flaw?

    Correct answer: Spring-brake effectiveness also depends on proper brake adjustment

    Spring force still acts through the wheel-brake mechanism. The manual warns that poor adjustment can impair both regular braking and spring-operated emergency or parking braking.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.14 - Spring Brakes; printed5-3, physical79. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  17. A driver familiar with a newer yellow push-pull parking knob is assigned an older air-braked truck with a labeled parking lever. What should guide the driver's use of it?

    Correct answer: The assigned truck's identified control and operating instructions

    The manual describes newer push-pull knobs and different controls on older vehicles. Drivers need to identify and learn the assigned vehicle's parking control, rather than transferring a shape-based assumption.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.1.15 - Parking Brake Controls; corroborating 5.4.8 on physical85/printed5-9; printed5-3, physical79. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  18. A straight truck has a dual air-brake system but only one service-brake pedal. A trainee thinks a second pedal must be missing. Which explanation resolves the concern?

    Correct answer: Separate air circuits can share a single set of brake controls

    A dual air system has separate air systems with their own tanks, hoses and lines, while using one set of brake controls. The manual describes typical axle assignments, not one universal layout.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.2 - Dual Air Brake (text, not the single-circuit Figure5.4); printed5-5, physical81. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  19. Before departure in a dual-air straight truck, one reservoir reading is normal but the other remains below the vehicle's required operating pressure. What should the driver do?

    Correct answer: Wait for both circuits to meet the operating requirements

    The manual directs the driver to watch both system readings and allow both to build adequate pressure before driving. A normal reading for one circuit does not verify the other; unexplained failure to build requires correction.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.2 - Dual Air Brake; printed5-5, physical81. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  20. A pre-trip check identifies excessive brake pushrod travel on a truck equipped with automatic slack adjusters. An unqualified driver proposes turning the adjuster until the reading looks acceptable. What is the better response?

    Correct answer: Arrange qualified diagnosis and correction of the underlying fault

    An automatic adjuster found out of adjustment may have a fault, an installation problem or related foundation-brake trouble. Manual resetting can mask the cause. Brake service must respect applicable qualifications and the manufacturer's procedures.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.2 - During Step 5 Walk-around Inspection > automatic slack adjusters; corroborating 49CFR396.25 and DTNA S2 article in audit; printed5-6, physical82. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  21. During the walk-around, a hose connected to a brake chamber has been rubbing against a bracket and is visibly worn. No hiss is heard at that moment. How should it be treated?

    Correct answer: As a brake-system defect needing attention before operation

    The air-brake inspection specifically includes chamber hoses that are cut or worn from rubbing. Absence of an audible leak at that moment does not make visible hose damage acceptable.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.2 - During Step 5 Walk-around Inspection > Brake Drums, Linings, and Hoses; printed5-6, physical82. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  22. A correctly conducted, safely secured pre-trip check confirms that a truck's low-air warning does not operate as specified. The supply gauge itself works. What conclusion follows?

    Correct answer: The failed warning needs correction despite the working gauge

    The warning is intended to alert the driver before depleted pressure severely limits braking. The gauge and warning have separate roles; the check has found a defect rather than an acceptable substitute arrangement.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.3 - Step 7 Final Air Brake Check > Test Low Pressure Warning Signal; equipment applicability corroborated by 49CFR393.51; printed5-6, physical82. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  23. A dual-air truck has larger-than-minimum reservoirs. Its manufacturer specifies a buildup time appropriate to that configuration. How should a trainee assess its pressure buildup?

    Correct answer: Use the applicable manufacturer specification for that equipment

    The manual explicitly notes that larger tanks may take longer to fill and directs the reader to manufacturer specifications. Equipment-specific context matters; this item does not set a numerical test or scoring limit.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.3 - Check Rate of Air Pressure Buildup; printed5-7, physical83. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  24. In the manual's safely secured reservoir-leakage check, the engine is shut off before the pressure loss is measured. What is the reason for measuring without the compressor adding air?

    Correct answer: It prevents replenishment from hiding the pressure loss

    A loss measurement must not be offset by fresh compressed air. The source distinguishes released and applied brake conditions and vehicle configurations; this conceptual item is not a complete test sequence or a leakage-limit question.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.3 - Test Air Leakage Rate; printed5-7, physical83. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  25. A completed check shows that a truck cannot build air pressure fast enough to meet the specification for its actual system. The route is short. What is the appropriate decision?

    Correct answer: Do not depart until the buildup problem is corrected

    Inadequate buildup can leave too little air for braking during driving. The manual says to have that problem fixed before driving; a short route does not remove the defect.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.3 - Check Rate of Air Pressure Buildup; printed5-7, physical83. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  26. Under the prescribed safe test conditions, a truck remains stationary during a gentle low-gear pull against its applied parking brake. What has that check established?

    Correct answer: The parking brake resisted that gentle holding check

    The gentle pull is a parking-brake holding check. It does not replace the separate service-brake, air-leakage or warning-device checks, and it is not a full assessment of braking performance.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.3.3 - Test Parking Brake; Test Service Brakes; printed5-7, physical83. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  27. A driver has already recognized a hazard and pressed the pedal, but time is still needed for air to travel through the system and apply the wheel brakes. Which part of stopping is being described?

    Correct answer: The air-brake lag after the pedal is pressed

    Brake lag is the air-system delay after pedal operation and before the brakes act. It is separate from the driver's perception and reaction intervals and must be included in stopping-distance planning.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.4.4 - Stopping Distance; Figure5.6 on physical85/printed5-9; printed5-8, physical84. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  28. A truck's drum brakes have become excessively hot from overuse, although reservoir pressure remains normal. Why can stopping effectiveness still decline?

    Correct answer: Heat can reduce lining friction and expand the drums

    Air pressure alone does not establish the condition of the friction brakes. Excessive heat can change the lining's friction and expand a drum, increasing shoe travel and reducing effective contact.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.4.5 - Brake Fading or Failure; printed5-9, physical85. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  29. Some brakes on a truck are out of adjustment and contribute less than their share during repeated braking. What additional problem can develop at the brakes still doing most of the work?

    Correct answer: They can overheat and fade from carrying more of the braking load

    The manual explains that brakes not doing their share leave the remaining brakes to do more work. Those brakes can overheat and fade, so one part of the system cannot simply compensate indefinitely for another.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.4.5 - Brake Fading or Failure; printed5-9, physical85. Current NCDMV-linked copy reviewed September 13, 2026. ↗

  30. While an air-braked straight truck is moving, its low-air warning comes on and pressure is falling. Service braking is still available. What is the safest objective?

    Correct answer: Reach a safe stop and park as soon as possible while control remains

    The manual directs the driver to stop and safely park promptly after a low-air warning. Automatic spring-brake application is not a safe stopping plan: remaining service-brake control is limited as the air supply falls.

    Source: North Carolina Commercial Driver License Manual, 2005 CDL Testing System, Version July 2014; Section 5 > 5.4.7 - Low Air Pressure; printed5-9, physical85. Current NCDMV-linked copy reviewed September 13, 2026. ↗

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Home / North Carolina / CDL

North Carolina CDL: Air Brakes

30 original questions on air-system parts, warnings, inspection concepts and equipment-specific precautions.

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The short sets use these same 30 records. This selection is not the official exam length or a complete skills course. Pressure settings and checking methods depend on the system and current requirements. These questions do not provide a complete inspection script, repair procedure or practical test. Use the official CDL manual, the instructions for your vehicle and required hands-on training.